The integrated immune response to Babesia microti of human origin in mice will be studied. Immunocytes from phenotypically normal and natural killer (NK) cell-defective beige mutant mice will be tested in vitro for their responses to parasite antigens. The effect of depletion of various lymphocyte subpopulations on the course of parasitemia due to B. microti will also be examined in these 2 strains of mice. Finally, the relative roles of immune T and B lymphocytes, NK cells and macrophages in immunologic memory of this parasite will be examined by adoptive transfer of selected cell types into beige nude mice and their heterozygous littermates. These studies should yield important information concerning the nature of the total immune response to this organism which could have potential application in immunoprophylaxis or immunotherapy of human and livestock babesiosis, malaria, and other diseases in which the host is threatened by the multiplication of intracellular organisms.